This invention relates to thermal stabilization of a multiple cavity structure, wherein cylindrical cavities are arranged coaxially in tandem, as in the construction of a microwave filter of plural resonant chambers, or cavities, and, more particularly, to an arrangement of multiple cavities employing transverse bowed walls with and without coupling apertures encircled by rings of material with differing coefficients of thermal expansion to provide selected ratios of thermally induced deformation of the transverse walls to counteract changes in resonance induced by thermal expansion/contraction of an outer cylindrical wall of the cavity structure.
Plural cavity structures are employed for microwave filters. A cavity which is frequently employed has the shape of a right circular cylinder wherein the diameter and the height (or the axial length) of the cavity together determine the value of a resonant frequency. For filters described mathematically as multiple pole filters, it is common practice to provide a cylindrical housing with transverse disc shaped partitions or walls defining the individual cavities. Irises in the partitions provide for coupling of desired modes of electromagnetic wave between the cavities to provide a desired filter function or response.
A problem arises in that changes in environmental temperature induce changes in the dimensions of the filter with a consequent shift in the resonant frequency of each filter section. For example, a filter fabricated of aluminum undergoes substantial dimensional changes as compared to a filter constructed of INVAR due to the much larger thermal coefficient of expansion for aluminum as compared to INVAR.
A solution to the foregoing problem, useful for a two-cavity filter is presented in U.S. Pat. No. 4,677,403 of Kich. Therein, an end wall of each cavity is formed of a bowed disc, while a central wall having an iris for coupling electromagnetic energy has a planar form. An increase of temperature enlarges the diameter of each cavity, and also increases the bowing of the end walls with a consequent reduction in the axial length of each cavity. The resonant frequency shift associated with the increased diameter is counterbalanced by the shift associated with the decrease in length. Similar compensation occurs during a reduction in temperature wherein the diameter decreases and the length increases.
The frequency stabilization provided by the foregoing patent is limited to the two-cavity filter having opposed thermal compensation end walls. However, there are filter situations requiring more complicated filter structure for higher pole and higher performance filters. Such filters may employ three or four cavities, by way of example, and there is a need to provide thermal compensation to such filters.